We are characterizing active streptokinase-plasminogen activator generated from the interaction of inactive streptokinase with plasminogen or plasmin. We are developing a technique by which the carboxyl groups of the active site of an enzyme (plasmin) can be specifically substituted with C14-labelled amino acid ester substrates (LMe) in presence of carbodiimide catalyst. We are studying the qualitative and quantitative requirements for the aggregation and retraction (fusion) of washed human platelets by human serum. We are studying patients with abnormalities of the coagulolytic system (hypercoagulability).